Optical fiber assembly, methods of manufacture thereof and articles comprising the same

ABSTRACT

Disclosed herein is an optical fiber assembly comprising a launching fiber having a receiving end and a transmitting end; an illuminating fiber having a receiving end and a transmitting end; where the receiving end of the launching fiber is operative to receive light from a light source and the transmitting end of the launching fiber is operative to transmit light to the receiving end of the illuminating fiber; where the launching fiber contacts the illuminating fiber in a manner so as to be offset from a center of a cross-sectional area of the illuminating fiber; and where the launching fiber has a diameter that is ⅛ to ½ of a diameter of the illuminating fiber; and a lens that is operative to contact the transmitting end of the illuminating fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/934,145 filed on Jan. 31, 2014 the entire contents of which arehereby incorporated by reference.

BACKGROUND

This disclosure relates to an optical fiber assembly, to methods ofmanufacture thereof and to articles comprising the same. In particular,this disclosure relates to an optical fiber assembly with a widedivergence angle and uniform light distribution, to methods ofmanufacture thereof and to articles comprising the same.

For optical fibers for illumination, the output divergence angle fromthe fiber is typically limited by its numerical aperture (NA). Forexample, using a 0.47 numerical aperture step index optical fiber, thefull width at half maximum (FWHM) divergence angle is approximately 56degrees. It is however desirable, to have wider divergence angles forsome illumination applications. In the past, a ball lens, a concave oraxicon lens was attached to the end of the fiber to increase the maximumdivergence angle. However in this configuration, the output distributionis no longer uniform and this non-uniform distribution is undesirablefor certain applications. There is therefore a need for obtaining animproved output divergence angle of greater than 56 degrees while at thesame time obtaining a uniform light distribution.

SUMMARY

Disclosed herein is an optical fiber assembly comprising a launchingfiber having a receiving end and a transmitting end; an illuminatingfiber having a receiving end and a transmitting end; where the receivingend of the launching fiber is operative to receive light from a lightsource and the transmitting end of the launching fiber is operative totransmit light to the receiving end of the illuminating fiber; where thelaunching fiber contacts the illuminating fiber in a manner so as to beoffset from a center of a cross-sectional area of the illuminatingfiber; and where the launching fiber has a diameter that is ⅛ to ½ of adiameter of the illuminating fiber; and a lens that is operative tocontact the transmitting end of the illuminating fiber; where theoptical fiber assembly transmits light received from the light source ata divergence angle of greater than 60 degrees.

Disclosed herein too is an optical fiber assembly comprising a launchingfiber having a receiving end and a transmitting end; an illuminatingfiber having a receiving end and a transmitting end; a mask disposedbetween the launching fiber and the receiving fiber; the mask having adiameter that is at least equal to a diameter of the launching fiber;where the mask comprises an opaque section and a transparent section;the transparent section having a circular cross-section that has adiameter that is ⅛ to ½ of a diameter of the illuminating fiber; wherethe transparent section has a center that is offset from a center of theilluminating fiber; and a lens that is operative to contact thetransmitting end of the illuminating fiber; where the optical fiberassembly transmits light received from the light source at a divergenceangle of greater than 60 degrees.

Disclosed herein too is a method of manufacturing an article comprisingdisposing a cladding upon an optical fiber assembly; where the opticalfiber assembly comprises a launching fiber having a receiving end and atransmitting end; an illuminating fiber having a receiving end and atransmitting end; where the receiving end of the launching fiber isoperative to receive light from a light source and the transmitting endof the launching fiber is operative to transmit light to the receivingend of the illuminating fiber; where the launching fiber contacts theilluminating fiber in a manner so as to be offset from a center of across-sectional area of the illuminating fiber; and where the launchingfiber has a diameter that is ⅛ to ½ of a diameter of the illuminatingfiber; and a lens that is operative to contact the transmitting end ofthe illuminating fiber; where the optical fiber assembly transmits lightreceived from the light source at a divergence angle of greater than 60degrees.

Disclosed herein too is a method of using an optical fiber assemblycomprising illuminating with electromagnetic radiation an optical fiberassembly comprising a launching fiber having a receiving end and atransmitting end; an illuminating fiber having a receiving end and atransmitting end; where the receiving end of the launching fiber isoperative to receive light from a light source and the transmitting endof the launching fiber is operative to transmit light to the receivingend of the illuminating fiber; where the launching fiber contacts theilluminating fiber in a manner so as to be offset from a center of across-sectional area of the illuminating fiber; and where the launchingfiber has a diameter that is ⅛ to ½ of a diameter of the illuminatingfiber; and a lens that is operative to contact the transmitting end ofthe illuminating fiber; where the optical fiber assembly transmits lightreceived from the light source at a divergence angle of greater than 60degrees; where the illuminating of the optical fiber assembly takesplace at the receiving end of the launching fiber.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(A) is a schematic that depicts one exemplary embodiment of theoptical fiber assembly that comprises a launching fiber in contact withthe illuminating fiber;

FIG. 1(B) is a schematic that depicts the individual optical parts ofthe optical fiber assembly;

FIG. 1(C) is a schematic that depicts another exemplary embodiment ofthe optical fiber assembly that comprises a launching fiber in contactwith the illuminating fiber;

FIG. 2 shows other exemplary positions where the circular transmittingend (shown in dotted lines and having a center N) of a single launchingfiber can contact the light receiving end of the illuminating fiber.Alternatively the circular dotted lines can embody locations where aplurality of circular transmitting ends of a plurality of launchingfibers can contact the receiving end of the illuminating fiber;

FIG. 3 shows one embodiment where a plurality of launching fibers arearranged to contact the cross-sectional area of the receiving end of theilluminating fiber;

FIG. 4 is a schematic of an exemplary embodiment depicting one method offacilitating contact between the transmitting end of the illuminatingfiber and the ball lens;

FIGS. 5(A)-5(E) depict a variety of different configurations for lensesthat may be used in the optical fiber assembly;

FIG. 6(A) depicts and exemplary embodiment of a mask that is disposedbetween the launching fiber and the illuminating fiber;

FIG. 6(B) depicts an exemplary embodiment of the mask which comprises anopaque portion and a transparent section through which light from thelaunching fiber may be transmitted to the illuminating fiber;

FIGS. 7(A) and 7(B) depict exemplary configurations for the locations ofthe transparent sections of the mask;

FIG. 8 shows the transparent section of the mask as a semi-circular slotthat results from the combination of several adjacent circulartransparent sections;

FIG. 9 depicts an exemplary embodiment of a circular transparent sectionin the mask; and

FIGS. 10(A) and 10(B) depict exemplary embodiments of assembled opticalfiber assemblies.

DETAILED DESCRIPTION

Disclosed herein is an optical fiber assembly that comprises a launchingfiber, an illuminating fiber and a lens. The launching fiber transmitslight into the illuminating fiber and into the lens, which then emitsthe light at a wide divergence angle with a uniform distribution. In oneembodiment, the launching fiber has a smaller diameter than theilluminating fiber and uses a restricted mode launching into theillumination fiber. In another embodiment, the launching fiber has adiameter equal to or larger than the illuminating fiber but uses arestricted mode launch into the illumination fiber. The dimensions ofthe launching fiber (at the point of electromagnetic radiationtransmission to the illuminating fiber) relative to the dimensions ofthe illuminating fiber facilitate the emission of electromagnetic raysfrom the optical fiber at a wide divergence angle (that is greater than56 degrees for a numerical aperture of 0.47) with a uniform intensitydistribution. Electromagnetic radiation as detailed can include allwavelengths in the electromagnetic spectrum. In a preferable embodiment,the electromagnetic radiation is visible radiation having wavelengths of300 to 750 nanometers, preferably 390 to 700 nanometers.

FIGS. 1(A) and 1(B) depict one embodiment of the optical fiber assembly100 that comprises a launching fiber 102 having a diameter d₁ in contactwith the illuminating fiber 104 having a diameter d₂. The illuminatingfiber 104 contacts a lens 106 through which light is emitted at a widedivergence angle with a uniform intensity distribution. The launchingfiber 102 has a diameter d₁ that is smaller than the diameter d₂ of theilluminating fiber 104. The diameter d₁ of the launching fiber and thediameter d₂ of the illuminating fiber 104 both exclude the thickness ofcladding that may be disposed on the fiber and represents the corediameter of the fibers respectively.

With reference now to the FIG. 1(b), the launching fiber 102 has areceiving end 101 (where it receives electromagnetic radiation) and atransmitting end 103 (from which the electromagnetic radiation istransmitted to the illuminating fiber). The illuminating fiber 104 has areceiving end 105, where it receives electromagnetic radiation from thetransmitting end 103 of the launching fiber 102 and a transmitting end107 from where it transmits electromagnetic radiation to the lens 106.

The launching fiber 102 contacts the illuminating fiber 104non-concentrically. In other words, a longitudinal axis X-X′ of thelaunching fiber 102 is always offset from a longitudinal axis Y-Y′ ofthe illuminating fiber 104 by a distance d. The longitudinal axis X-X′of the fiber 102 does not coincide with the longitudinal axis Y-Y′ ofthe illuminating fiber 104 in the optical fiber assembly 100. Thelaunching fiber 102 can contact the illuminating fiber 104 at any pointon its surface except where their respective longitudinal axes coincidewith one another. The offset distance “d” is ⅛ to ½ of the distance d₂.

As seen in the FIG. 1(A), a longitudinal axis of the launching fiber isparallel to a longitudinal axis of the illuminating fiber at a point ofcontact of the launching fiber with the illuminating fiber. In anotherembodiment depicted in the FIG. 1(C), a longitudinal axis of thelaunching fiber is inclined to a longitudinal axis of the illuminatingfiber at a point of contact of the launching fiber with the illuminatingfiber. In the FIG. 1(C) it may be seen that the launching fiber isinclined at an angle α to the illuminating fiber. The angle α shouldgenerally be less than the acceptance angle of the illuminating fiber.In a preferred embodiment, the angle α has a value of 5 to 30 degrees.

FIG. 2 shows other exemplary positions where the circular transmittingend 103 (shown in dotted lines and having a center N) of a singlelaunching fiber 102 can contact the light receiving end 105 of theilluminating fiber 104 (represented by the solid line and having acenter M). The dotted lines represent the various positions that asingle launching fiber 102 can contact the illuminating fiber 104. Ascan be seen in the FIG. 2, the launching fiber 102 can contact theilluminating fiber such that a portion of its outer periphery orcircumference contacts the outer periphery or circumference of theilluminating fiber 104. In another embodiment, the launching fiber 102can contact the illuminating fiber such that the center M and N (of theilluminating fiber and the launching fiber respectively) can lie withinthe area represented by the circular transmitting end 103 of thelaunching fiber so long as these centers do not coincide with oneanother.

In a preferred embodiment, the launching fiber 102 contacts theilluminating fiber 104 in a manner such that at least one tangent to theouter circumference of the launching fiber 102 coincides with at leastone tangent to the outer circumference of the illuminating fiber 104. Inshort, at least a portion of the outer circumference of the launchingfiber 102 coincides with a portion of the outer circumference of theilluminating fiber 104.

In another embodiment, a plurality of launching fibers 102 may beperiodically or aperiodically arranged within the periphery of thereceiving end (i.e., within the cross-sectional area of the receivingend) of the illuminating fiber 104. FIG. 3 shows one embodiment where aplurality of launching fibers 102 are arranged within thecross-sectional area of the receiving end of the illuminating fiber 104.As may be seen in the FIG. 3, the launching fibers 102 have a fixedperiodicity between them. The launching fibers 102 may also be arrangedto randomly (such that there is no periodicity between them) contact thecross-sectional area of the receiving end of the illuminating fiber 104.The FIG. 2 can be alternatively viewed as a fiber assembly 100 where 5launching fibers 102 are randomly arranged to contact a singleilluminating fiber 104.

In another embodiment (not shown), the fiber assembly 100 may comprise aplurality of launching fibers 102 where each launching fiber may have adifferent diameter. For example, a first launching fiber 102 having adiameter d₂, a second launching fiber d₃ and a third launching fiber d₄(where d₂ is greater than d₃, which is greater than d₄) can each contacta single illuminating fiber to emit light at a wide divergence anglewith a uniform distribution.

It is desirable for the launching fibers 102 to have an outer corediameter d₂ of ⅛ to ½, of the diameter of the illuminating fiber 104. Inanother embodiment, the launching fibers 102 have an outer core diameterd₂ of ⅙ to ⅓ of the diameter of the illuminating fiber 104. It is alsodesirable for the launching fiber to have a numerical aperture that issmaller than the numerical aperture of the illuminating fiber. In oneembodiment, the numerical aperture of the launching fiber is 0.12 to0.20 at a wavelength of 850 nanometers. In another embodiment, thenumerical aperture of the launching fiber is 0.14 to 0.18 at awavelength of 850 nanometers.

The length of the launching fiber is long enough to reduce any speckles.The length of the launching fiber is L_(c)/(NA)2, where L_(c) is thecoherence length of the light source and where NA is the numericalaperture of the launching fiber 102.

The illuminating fiber 104 can be any fiber that is used for thetransmission of electromagnetic radiation.

The lens 106 can be a ball lens, a concave or axicon lens or ahemispherical lens. Any suitable size of the lens may be used to divertthe output from the illumination fiber. In an embodiment, a suitablediameter of the lens is approximately the same to twice the diameter ofthe core of the illumination optical fiber. In another embodiment, thelens is ball lens having a diameter of approximately 500 micrometers orless. Any suitable method may be used to attach the lens to the endportion of the illumination optical fiber. For example, the end portionof the illumination fiber and the ball lens may be inserted into a tubesuch as a glass tube as shown in FIG. 4.

With reference now to the FIG. 4, it may be seen that the illuminatingfiber 104 contacts a ball lens 106 at the transmitting end of theilluminating fiber 104. The illuminating fiber 104 has a cladding 108disposed upon it. A tube 110 contacts the cladding 108 as well as theball lens 106 and secures the ball lens 106 in such a manner as tocontinuously contact the illuminating fiber 104. The tube 110 maycomprise a glass, a ceramic, a metal or a polymer. In an exemplaryembodiment, the tube 110 may comprise an elastomer. An elastomer permitsthe lens 106 to be replaced as desired (with larger or smaller lens ifdesired).

In addition to ball lens, concave or axicon lens, there are a variety ofother lens that may be used. FIGS. 5(A)-5(E) depict a variety ofdifferent configurations for lenses 106 that may be used in the opticalfiber assembly 100. The lens 106 have a first surface 106B (thatpreferably receives the electromagnetic radiation) and a second surface106A (that preferably transmits the electromagnetic radiation). In oneembodiment, the first surface 106B contacts the illuminating fiber atthe transmitting end. While the aforementioned configurations state thatthe first surface 106B contacts the transmitting end of the illuminatingfiber, there may be instances where the second surface 106A is thereceiving surface for the electromagnetic radiation and in this case thefirst surface 106B acts as the transmitting surface of the lens. FIG.5(A) depicts a triangular lens, FIG. 5(B) represents a hemisphericallens, FIG. 5(C) represents a crescent shaped lens, FIG. 5(D) representsa lens having ¾ the area of a circular lens and the FIG. 5(E) representsa lens having a D-shape. The curved face 106A of the D-shaped lens mayrange from elliptical surface to a circular surface. In other words, thelocus of points that define the surface 106A of the lens of the FIG.5(E) may be a part of a circle or part of an ellipse.

As noted above, in one configuration, the launching fiber may have adiameter that is equal to the diameter of the illuminating fiber and yettransmit electromagnetic radiation in restricted modes to theilluminating fiber from the launching fiber. An example of thisconfiguration is displayed in the FIG. 6(A). In the FIG. 6(A), a mask112 is disposed between the launching fiber 102 and the illuminatingfiber 104. When viewed from the direction 200 of the FIG. 6(A), the mask112 as seen in the FIG. 6(B) comprises an opaque portion 114 and atransparent section 116 through which light from the launching fiber 102may be transmitted to the illuminating fiber 104. A lens 106 contactsthe transmitting end of the illuminating fiber 104. The mask 112 has acircular cross-sectional area whose diameter is greater than or equal tothe diameter of the launching fiber. Its diameter is also greater thanor equal to the diameter of the illuminating fiber.

The mask 112 contacts the transmitting end of the launching fiber andthe receiving end of the illuminating fiber. In one embodiment, thetransparent section 116 has a circular cross-sectional area whosediameter is ⅛ to ½, preferably ⅙ to ⅓ of the diameter of theilluminating fiber 104. The transparent section 116 is always offsetfrom the center of the illuminating fiber 104. The center point of thetransparent section 116 never coincides with the center of thecross-sectional area of the illuminating fiber 104. In an embodiment,the center of the transparent section 116 is offset from the center ofthe illuminating fiber 104 by a distance that is ⅛ to ½ of the diameterof the illuminating fiber 104.

The transparent section 116 may comprise air, a transparent sectionhaving the same refractive index as that of the launching fiber or theilluminating fiber or having a refractive index that is greater thanthat of the launching fiber but less than that that of the illuminatingfiber.

The mask 112 may have more than one transparent section 116 as seen inthe FIGS. 7(A) and 7(B). In one embodiment, the mask may have aplurality of transparent sections each of which are offset from thecenter of the illuminating fiber. The FIGS. 7(A) and 7(B) are replicasof the FIGS. 2 and 3 respectively. In short, the transparent section 116of the mask 112 can be located at the same positions that the launchingfiber 102 contacts the illuminating fiber 104 as depicted in the FIGS. 2and 3 respectively.

In one embodiment, some of the plurality of transparent sections 116 (asseen in the FIG. 7(B)) can be combined to form a slot through whichlight can enter from the launching fiber and be transmitted to theilluminating fiber. This is depicted in the FIG. 8 which shows thetransparent section 116 as a semi-circular slot that results from thecombination of several adjacent circular transparent sections 116A,116B, 116C, 116D and 116E. The slots can be distributed in differentportions of the mask 112 (not shown). The slot can also be circular(i.e., it can be parallel to the entire outer circumference of theilluminating fiber 104 as can be seen in the FIG. 9.

The mask 112 can be manufactured from any material that is opaque and asection may then be drilled or etched therethrough to create thetransparent section 116. The mask 112 may comprise a metal, a ceramic ora polymer. In an exemplary embodiment, a block copolymer havingspherical domains may be etched and used as the mask 112.

The optical fiber assembly may be illuminated by a laser. Suitablelasers are diode lasers, frequency double solid state lasers or gaslasers.

The FIGS. 10(A) and 10(B) depict the entire optical fiber assembly withcladding disposed thereon. With reference now to the FIGS. 10(A) and10(B), in one embodiment, in one method of manufacturing the opticalfiber assembly 100, a cladding 108 is disposed so as to contact thelaunching fiber 102, the mask 112 if present, and the illuminating fiber104. The lens 106 can also be disposed into the cladding if desired.

In another embodiment, previous detailed in the FIG. 4, a separate tube110 may be disposed on the cladding 108 to contact the lens and to holdit in position. The cladding may be disposed on the respective fibers byextrusion. In an exemplary embodiment, crosshead extrusion may be usedto dispose the cladding on the launching fibers and the illuminatingfibers.

The cladding is generally manufactured from a polymer that has arefractive index that is lower than that of the launching andilluminating fibers. In an exemplary embodiment, the cladding comprisesethylene tetrafluoroethylene, (ETFE), a fluorine based plastic, whilethe illuminating and launching fiber each comprise silica.

The dimensions of the launching fiber (at the point of electromagneticradiation transmission to the illuminating fiber) relative to thedimensions of the illuminating fiber facilitate the emission ofelectromagnetic rays from the optical fiber at a wide divergence angle(that is greater than 60 degrees, preferably greater than 65 degrees andmore preferably greater than 70 degrees for a numerical aperture of0.47) with a uniform intensity distribution. The optical fiber assemblyis advantageous in that its divergence angle exceeds the full width athalf maximum of the divergence angle of the illumination fiberilluminated and measured under the same conditions with the same typeand size of lens.

It is to be noted that all ranges detailed herein include the endpoints.Numerical values from different ranges are combinable.

The transition term comprising encompasses the transition terms“consisting of” and “consisting essentially of”.

The term “and/or” includes both “and” as well as “or”. For example, “Aand/or B” is interpreted to be A, B, or A and B.

While the invention has been described with reference to someembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing fromessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. An optical fiber assembly comprising: a launchingfiber having a receiving end and a transmitting end; an illuminatingfiber having a receiving end and a transmitting end; where the receivingend of the launching fiber is operative to receive light from a lightsource and the transmitting end of the launching fiber is operative totransmit light to the receiving end of the illuminating fiber; where thelaunching fiber contacts the illuminating fiber in a manner so as to beoffset from a center of a cross-sectional area of the illuminatingfiber; and where the launching fiber has a diameter that is ⅛ to ½ of adiameter of the illuminating fiber; and a lens that is operative tocontact the transmitting end of the illuminating fiber; where theoptical fiber assembly transmits light received from the light source ata divergence angle of greater than 60 degrees; where the launching fiberhas a length of Lc/(NA)², where Lc is the coherence length of the lightsource and where NA is the numerical aperture of the single launchingfiber.
 2. The optical fiber assembly of claim 1, where the launchingfiber has a numerical aperture of 0.12 to 0.20 at a wavelength of 850nm.
 3. The optical fiber assembly of claim 1, where the launching fiberhas a diameter that is ⅙ to ⅓ of the diameter of the illuminating fiber.4. The optical fiber assembly of claim 1, where the divergence angleexceeds the full width at half maximum of the divergence angle of theillumination fiber measured under the same conditions.
 5. The opticalfiber assembly of claim 1, where the optical fiber assembly transmitslight received from the light source at a divergence angle of greaterthan 60 degrees and where an intensity distribution is uniform acrossthe entire divergence angle.
 6. The optical fiber assembly of claim 1,where the launching fiber and the illuminating fiber each comprise asilica core.
 7. The optical fiber assembly of claim 1, wherein a tangentto an outer circumference of the launching fiber coincides with atangent to an outer circumference of the illuminating fiber.
 8. Theoptical fiber assembly of claim 1, wherein a longitudinal axis of thesingle launching fiber is parallel to a longitudinal axis of theilluminating fiber at a point of contact of the launching fiber with theilluminating fiber.
 9. The optical fiber assembly of claim 1, wherein alongitudinal axis of the launching fiber is inclined to a longitudinalaxis of the illuminating fiber at a point of contact of the launchingfiber with the illuminating fiber.
 10. The optical fiber assembly ofclaim 9, wherein an angle of inclination between the longitudinal axisof the launching fiber and the longitudinal axis of the illuminatingfiber is less than an angle of acceptance of the illuminating fiber. 11.The optical fiber assembly of claim 1, wherein the lens is a ball lens,a concave lens or an axicon lens.
 12. A method of manufacturing anarticle comprising: disposing a cladding upon an optical fiber assembly;where the optical fiber assembly comprises: a launching fiber having areceiving end and a transmitting end; an illuminating fiber having areceiving end and a transmitting end; where the receiving end of thelaunching fiber is operative to receive light from a light source andthe transmitting end of the launching fiber is operative to transmitlight to the receiving end of the illuminating fiber; where thelaunching fiber contacts the illuminating fiber in a manner so as to beoffset from a center of a cross-sectional area of the illuminatingfiber; and where the launching fiber has a diameter that is ⅛ to ½ of adiameter of the illuminating fiber; and a lens that is operative tocontact the transmitting end of the illuminating fiber; where theoptical fiber assembly transmits light received from the light source ata divergence angle of greater than 60 degrees; where the launching fiberhas a length of Lc/(NA)², where Lc is the coherence length of the lightsource and where NA is the numerical aperture of the single launchingfiber.
 13. A method of using an optical fiber assembly comprising:illuminating with electromagnetic radiation an optical fiber assemblycomprising: a launching fiber having a receiving end and a transmittingend; an illuminating fiber having a receiving end and a transmittingend; where the receiving end of the launching fiber is operative toreceive light from a light source and the transmitting end of thelaunching fiber is operative to transmit light to the receiving end ofthe illuminating fiber; where the launching fiber contacts theilluminating fiber in a manner so as to be offset from a center of across-sectional area of the illuminating fiber; and where the launchingfiber has a diameter that is ⅛ to ½ of a diameter of the illuminatingfiber; and a lens that is operative to contact the transmitting end ofthe illuminating fiber; where the optical fiber assembly transmits lightreceived from the light source at a divergence angle of greater than 60degrees; where the illuminating of the optical fiber assembly takesplace at the receiving end of the launching fiber; where the launchingfiber has a length of Lc/(NA)² where Lc is the coherence length of thelight source and where NA is the numerical aperture of the singlelaunching fiber.
 14. The method of claim 13, where the electromagneticradiation is visible light obtained from a laser source.